1. Field of the Invention
The present invention relates in general to vacuum pumps, such as turbo-molecular pumps, and relates in particular to a vacuum pumping apparatus which does not require cooling water for cooling the bearing and motor sections of the apparatus, and effectively prevents or reduces accumulation of reaction products.
2. Technical Background
FIG. 7 shows a cross sectional view of a turbo-molecular pump as an example of the conventional vacuum pump. The turbo-molecular pump is an axial flow pump and comprises a multi-stage assembly of moving vanes 4 rotating at high speed surrounded by stator vanes 4A. Gas molecules entering the pump through an inlet opening 1 strike and reflect off the surfaces of the moving vanes and stator vanes, producing a directed stream of exhaust molecules. Although this type of pump is capable of producing ultra-high vacuum, it is necessary to rotate the impeller (rotation body) 3 at a high speed of several tens of thousand revolutions per minute; and therefore, the supporting bearings must be non-lubricated magnetic bearings for ease of maintenance and prevention of gas generation.
As shown in FIG. 7 more specifically, the main shaft 2 having an impeller 3 fixedly attached thereto is supported in the radial direction by an upper radial bearing 9 and a lower radial bearing 11; and is supported in the axial direction by an upper axial bearing 12 and a lower axial bearing 13. These bearing components comprise coil 9A, 11A, 12A and 13A wound on respective yokes, and when supplied with electric current, interact magnetically with the opposing magnetic poles (not shown) of the main shaft 2. The driving motor 10 for driving the main shaft 2 to rotate at high speed comprises an induction or synchronous motor having coils 10A wound on a yoke. The driving motor 10 is supplied with an alternating current of relatively high frequency from a power source (not shown) which generates rotating spatial magnetic fields to rotate the magnetic poles of the main shaft 2. The static component comprises a driving motor section and the bearing component sections including of magnetic yokes and coils wound thereon, where the coils 10A, and the like, are coated with a resin 6, such as an epoxy resin, to protect the coils and prevent gas discharge. Connector 14 is provided to supply electrical power to the magnetic bearings 9, 11, 12 and 13 and the motor section 10.
The turbo-molecular pump presented above consumes a fairly large amount of operating power to drive the main shaft 2 at a high speed and to operate the magnetic bearings. For this reason, depending on the gas loading on the pump and operating temperatures, the coils can become over-heated, and unless a cooling facility is provided, the epoxy coating 6 on the motor section and the magnetic bearings tend to degrade with use and ultimately lead to a breakdown of the electrical insulation of the coils. The cooling facility is provided by a housing 7' and cooling pipes 15 for circulating cooling water around the motor section and the magnetic bearings, as shown in FIG. 7. However, it is cumbersome to operate such a cooling facility because of the necessity for securing cooling water and controlling its flow rate, and the cooling problem can be further aggravated by such operating problems as corrosion of the pipes, leaking of cooling water and blockage in the pipes. Therefore, the conventional pumping apparatus is unsatisfactory from the viewpoints of utility and reliability.
Further operating problems in the conventional pump can arise from the fact that, when the exhaust gas is being routed from the inlet opening 1 to the outlet opening 16, because the exhaust gas from the pump sometimes contains reaction products, some reaction products can sublime and deposit on the stator side surfaces A, B. As the process of deposition continues, the deposits accumulating on the impeller 3 lead to operational problems such as narrowing, or even closure, of the gas passage, or back streaming of the reaction products towards the inlet opening 1.
FIG. 8 shows the subliming behavior chart of one typical example of such reaction products, AlCl.sub.3. The x-axis represents temperature in celsius and the y-axis represents the vapor pressure in torr. It shows that AlCl.sub.3 is solid above the sublimation line while it is vapor below the line.
Vacuum exhaust gas containing AlCl.sub.3 entering the inlet opening 1 of the turbo-molecular pump is compressed between the turbine blades 4 of the impeller 3 and the screw groove 5 to be exhausted from the outlet opening 16. The exhaust gas increases its internal pressure as it approaches the outlet opening 16, often reaching a pressure between 0.01 to 3 torr, depending on the operating condition of the turbo-molecular pump. In this case, it can be seen from FIG. 8 that, at temperatures near room temperature, reaction products, which were originally in the vapor phase, can precipitate out as solid phase products. In practice, deposition occurs on the inner surface A of the spacer 8, the bottom surface B of the housing 7' and in the vicinity of the gas passage C. To prevent the occurrence of such deposition, it is necessary to increase the vapor pressure of the reaction products by increasing the temperature in the depositing regions, but in conventional pumps, these regions are cooled, in reality, by the cooling pipe 15 provided for cooling of the magnetic bearing sections and motor section.
As an approach to preventing deposition of reaction products, it is conceivable to dispose an external heater 24 on the housing 7' as shown in FIG. 9. In this case, because of the effect of the cooling water flowing through the cooling pipe 15, it is necessary to expend considerable amount of electrical power to heat the reaction products to the temperatures required to induce their sublimation. There is also a concern that the heater 24 may raise the temperature in the magnetic bearing sections and the motor section, thus leading to an overall assessment that this approach is not effective from the viewpoints of energy saving and reliability.